I've had my P516 7.5" pistol PDW for a while now, and if I'm honest I'm starting to get that itch for something other than 5.56. Part of the reason I purchased an AR platform is the modularity of the upper assemblies - I like the idea of being able to switch calibers/possible uses of the platform just by swapping out for a new upper receiver. Before I start looking at which one(s) to get though, I was wondering if there's anything regarding the 516 being a piston-driven design that would limit my selection of what other uppers might be compatible? If I'm honest I'd love to look into a .458 pistol upper, probably a .300 BLK and possibly even a .410 upper (if I can find one). Anything I need to know about before giving my debit card a workout?

A 516 lower should accommodate any standard AR15 upper assembly. :)

Awesome, thanks!

After a little research I think I've settled on getting a KAK 8" .458 upper to give my little PDW a big bite - Will I need to change out the buffer spring for a heavier duty one (currently has the stock 5.56 pistol buffer)?